With the gradual development of technology and affluent society, automobiles are available to every household for bringing convenience of traffic as well as the convenience is also gradually improved due to the progress of the automobile technology. However, with the popularizing of the utilization of automobiles, the casualties caused by automobile accidents are getting higher and higher. According statistics over the years published by ministry of the Interior of R.O.C, the number of death due to the automobile accidents is around 3000 each year in Taiwan area while the injured people are around 200,000. For example, in year 2005, the number of death and injured persons are 2,894 and 200,009 respectively, i.e. an average of 8 persons died daily due to the traffic accidents.
Analyzing the cause of accidents, it reveals that 97% of automobile accidents are caused by inappropriate driving behavior, and the most frequent type of automobile accident is the rear-end collision. The accidents not only cause mental or physical burdens to each individual or family but also consume the limited medical resources of the society and cause the loss of productivity of the country. In regard to this, if it is possible for building a collision warning system by image acquiring apparatus with advanced image processing techniques for identifying the other vehicles around and estimating the relative distance therebetween and issuing an alarm signal while the relative distance is too close so that the driver is capable of proceeding an appropriate operation such as lowing speed for preventing automobile accidents from being occurred and reducing incidence rate of automobile accidents.
However, while estimating the relative distance between the carrier and other vehicles around, it is necessary to obtain the angular information between the image acquiring apparatus and ground. The conventional art like U.S. Pat. No. 4,942,533 provided a technique for detecting a relative distance to a second vehicle at a position in a monitoring direction in front of the first vehicle. A vanishing point is determined by extracting, for example, the continuous or intermittently continuous white lines dividing traffic lanes on the road on the basis of spatial changes in the features of the scene ahead of the first vehicle, and the tilt angle of the optical axis of an optical picture information generating device such as a TV camera is obtained on the basis of the determined vanishing point to thereby detect the distance to a designated position in which the second vehicle will appear.